1. Field of the Invention
The present invention relates to an apparatus for forming a semiconductor crystal. More particularly, it relates to an apparatus for growing an epitaxial layer on a semiconductor substrate.
2. Description of Prior Art
Cleaning of a substrate surface is essential to grow a high quality single crystal at a low temperature. To accomplish this object, a method has been employed conventionally which forms an oxide film for protecting the substrate surface from contamination on a substrate by use of a chemical cleaning method, and then heats the substrate under ultra-high vacuum to remove the oxide film and to expose the cleaned surface. Cleaning of the surface used to be practiced by use of a vacuum vessel for the crystal growth inside a system as shown in FIG. 1. Referring now to FIG. 1, reference numeral 101 represents a preparation chamber; 102 is a gate valve; 106 and 107 are evacuating units, respectively; 105 is a gas introduction nozzle; 104 is a heater for heating a substrate; and 103 is a growth chamber. Refer, for example, to "Semiconductor Research", Vol. 21, No.5, pp. 101-116 published by Kogyo Chosakai, for an example of such a method.
In a mass-production process wherein cleaning of the surface and crystal growth are repeated inside the same system, the following problems occur if the surface cleaning step and the crystal growth step by an epitaxial growth method are carried out inside the same vacuum vessel. Namely, reaction products at the time of growth attach to the peripheral portions of a substrate heating system inside the vacuum vessel and its wall surface after the epitaxial growth, but since the substrate temperature is generally higher at the time of surface cleaning than at the time of epitaxial growth, the residual components that attach at the time of the epitaxial growth are emitted once again as a gas as shown in FIG. 2 due to thermal influences at a higher substrate temperature at the next cleaning step of the substrate surface and deteriorate the background inside the growth chamber. If the surface is cleaned under such a condition, cleaning of the substrate surface is not sufficient and the quality of the crystal that has been grown drops eventually. In order to avoid this problem in the conventional system, the vacuum chamber after growth must be exhausted for a long time while being baked before carrying out the surface cleaning step of the next substrate, and consequently, mass-producibility of the system drops.
Furthermore, in the case of manufacturing an insulated gate field effect transistor (hereinafter called "MOSFET") having an epitaxial growth layer grown in the growth chamber, the method for manufacturing the MOSFET as shown in FIGS. 3A and 3B has been employed. FIGS. 3A and 3B are sectional views showing the formation steps of an epitaxial growth layer 303 and a gate oxide film 304 in the fabrication process of a MOSFET in accordance with the prior art technique. As shown in FIGS. 3A and 3B, the epitaxial growth layer 303 which forms a channel region is formed on a substrate 301 by use of a semiconductor crystal growth apparatus having a growth chamber and then a gate oxide film 304 is formed on the epitaxial growth layer 303 by use of a thermal oxidation furnace through a chemical washing step. In FIG. 3, the reference numeral 302 indicates a field oxide film.
Generally, it is of importance to improve the interface condition between the channel region and the gate oxide film in order to improve MOSFET performance. In accordance with the prior art technique, however, the formation of the channel region and the formation of the gate oxide are carried out in entirely different systems as described above, and moreover chemical washing is conducted before the formation of the gate oxide film. Therefore, it is likely that impurity ions such as Na.sup.+ attach to the channel surface during or after washing or they are incorporated into the oxide film during the thermal oxidation step. This is one of the significant factors for deteriorating the interface condition between the channel and the gate oxide film or the quality of the oxide film.